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Tsunami Definition

Definition: A tsunami is a series of long-period sea
waves, of either local or distant origin, resulting from a sudden and
massive displacement of ocean water. Most often generated by a
large-scale seafloor displacement associated with a large-magnitude
earthquake, a tsunami can also result from a major submarine
landslide,
the catastrophic explosion of a volcanic island, or a significant impact
event.

Tsunami, a concatenation of two Japanese words, translates as
“harbor wave”. Tsunamis are often colloquially referred to
as tidal waves; this description is inaccurate because their
origins are not related to tides, although tidal conditions such as
diurnal variation (daily fluctuation of high versus low tide), lunar
maximums and wind- or storm-induced surge) can amplify their
magnitude and destructive capability. A tsunami may also be called a
seismic sea wave; this usage is appropriate if the tsunami has
been produced by an undersea earthquake.

Tsunami Facts & Info

Some typical tsunami facts and information:

A tsunami can only occur if there is a massive displacement of ocean
water. A major earthquake occurring onshore cannot produce a tsunami
unless a portion of the earthquake fracture zone results in an
offshore seafloor displacement or a submarine landslide is triggered
by the earthshaking.

Not all undersea earthquakes produce tsunamis. An earthquake must
exhibit a vertical component of seafloor movement in order to
produce a tsunami. A thrust earthquake is the type of quake
most likely to produce a tsunami.

Two major characteristics of an earthquake that may lead to the
issuance of a tsunami warning are its location and magnitude.
A critical characteristic which cannot be fully factored into a
tsunami warning is the type of earthquake because that
information is not immediately available. (The type of earthquake
most likely to occur on a particular fault system is often
known, however.)

It is important to take all tsunami warnings seriously, even
when prior warnings may not have been followed by a (visible) tsunami.
Remember that a tsunami warning alerts the public to the potential
for a dangerous tsunami; therefore, every warning should be
heeded. Numerous “baby” tsunamis occur which are
never noticed by beachgoers or coastal residents.

A tsunami is always a series of waves; these waves radiate
outward from the source until they intersect a landmass such as an
island or a coastline. Because the waves are of long period, a given
location experiencing a tsunami event may remain at risk for repeated
strikes for many hours. The first wave in the series is not
necessarily the largest wave.

Most tsunami waves in the deep water of the open ocean do not present a
threat because the wave height is generally under one meter. Even a
tsunami produced by a great earthquake, while possibly achieving an
open-ocean height of 15 meters, should not be very hazardous in deep
water because the long period of its waves will result in large swells
rather than bores or breaking waves. Tsunamis are most dangerous near
coastlines.

As tsunami waves encounter a coastline or shallow water, they undergo
a process known as shoaling. A shoaling wave slows down; as
it does so, the distance between the wave and the incoming wave behind
it (the wavelength) shortens, but the volume of water carried by
the waves remains constant. This “bunching” of waves,
the rising shoreline, and the momentum of the wave all serve to force
the water in a tsunami wave forward and upward as it approaches land.
Wave heights of large tsunamis typically range from 7 to 15 meters
(21 to 45 feet), but can reach heights of over 33 meters (100 feet).

It is important to note that, because tsunamis are a series of waves,
there arrival may result in a permanently elevated sea level
for the duration of the event. This phenomenon is similar to the
storm surge associated with
tropical
storms.

Tsunami waves are of greatest risk along coastlines, but may also pose
a significant risk in inlets and harbors and are capable of propagating
upriver from a coastline for appreciable distances. Low-lying areas
away from the beach may be at special risk because the water deposited
by a tsunami wave may have no way of returning to its source, leaving
such areas flooded by seawater.

Tsunami-like events can occur in smaller inland bodies of water such as fiords
or dammed lakes. These are not labeled as tsunamis, but possess
characteristics of a tsunami on a smaller scale (though the resultant wave
heights may be much greater). Typically, such events are singularly impact
events caused by landslide
or glacial calving. In reality, they resemble
mega-tsunamis, described below.

Mega-Tsunamis

Historically, giant tsunamis have resulted from significant impact events such
as the massive flank failure of an island stratovolcano or the collision
with the ocean surface of an extraterrestrial body such as a large meteor,
small asteroid or comet. These types of events may comprise both impact
and seafloor displacement components. The impact component is of special
interest because it can generate a shock wave capable of displacing
a far greater volume of water than would be expected solely from a
comparable displacement event. Such giant waves, which have in the past
been documented to have reached heights of at least 350 meters (over 1000
feet), are known as mega-tsunamis.

While there are at least two geologic areas of concern today that possess a
potential to create a mega-tsunami, potential does not necessarily
translate into immediate risk. Mega-tsunamis are geologically rare events,
occurring on the order of tens of thousands to millions of years. The
locations in question, in the Canary Islands in the Atlantic and the
Hawaiian Islands in the Pacific, present some compelling evidence as to
why a collapse could occur in the near-future, but present equally-compelling
reasons as to why it will not occur. These locations are being actively
monitored, so risk can be considered minimal at the present time. For an
excellent discussion of the potential mega-tsunami hazard posed by the
island stratovolcanoes of La Palma in the Canaries and on the Big Island
of Hawaii, read the
paper by George Pararas-Carayannis.

Tsunami Hazard Summary

In summary, tsunamis are hydrokinetic events that take place routinely around
the globe and present a hazard to anyone dwelling on or near a coastline,
especially around the Pacific Rim. Recent events have shown that regions
outside the Pacific Basin are not immune to the effects of tsunamis.
Large tsunamis are infrequent events, but the risk of a dangerous tsunami
is real and ever-present. Coastal inhabitants should be aware of tsunami
risk and should know the proper safety precautions to take if a tsunami
warning is issued.

Authored by Kenneth L. Anderson.
Original article published 18 February 2005, updated 19
February 2005.

Follow links to the right to learn more about tsunamis, what causes a tsunami, tsunami articles,
tsunamis and earthquakes, mega-tsunamis, tsunami history and tsunami
effects.
At the left margin, Related Links address topics of interest
pertaining to geologic hazards and other security issues. View the
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for a complete list of security and consumer protection topics.

ITIC - International Tsunami Information CentreInternational Coordination Group for the
Tsunami Warning System in the Pacific, (ICG/ITSU)
Intergovernmental Oceanographic Commission (IOC),
United Nations Educational,
Scientific and Cultural Organization (UNESCO),
United Nations (UN)
with the National Weather Service (NWS),
National Oceanic and Atmospheric
Administration (NOAA)